In The Adventure of the Speckled Band why did Helen not hear the whistle from when her sister died until the night before she went to see Mr. Holmes?

Helen's mother used to receive a sum of 1000 pounds a year
which after her death passed on to her family that consisted of Helen and her twin
sister Julia, and their step father. According to the terms of the mother's will once
the sisters got married a certain amount was to be given to them
annually.

The step father was aware of this and as he had
no other source of income of his own did all that he could to prevent the sisters from
getting married. He killed Julia with the help of his pet snake when he heard that she
was going to get married.

The snake had been trained to
respond to the sound of his whistle and return to him, once he had bitten the victim.
After Julia died there was no need to use the whistle to recall the
snake.

Later, when Helen decided to get married, the step
father tried to kill Helen and attempted the same method he had used with Julia. This
required him to blow the whistle.

Fortunately for Helen, he
did not succeed the first time; the whistle's sound instead warning Helen and leading
her to seek the help of Sherlock Holmes.